Efficient multiplexed transmission of digital speech and data signals in digital transmission systems requires encoding of the signals, prior to transmission over a transmission line. While there exist several encoding techniques, the encoding technique to which the present invention is directed is known as adaptive vector quantization. In adaptive vector quantization, the best fit for the pattern of each group of samples of successive groups of samples of pulse code modulated (PCM) encoded speech signals is found corresponding to previously stored groups of patterns in a code book, that is, in a memory which stores predetermined digital signal sequences of expected sample group patterns. Upon finding a best match, the entry number of the stored pattern in the memory is transmitted, rather than the pattern (the digital signal sequences) itself. At a decoder, a signal corresponding to the entry number of the best match pattern is applied to a code book memory in which is stored sample patterns identical to that in the code book memory of the encoder. Once the entry number has been entered, the pattern (or vector) signal is output and is and converted to a PCM signal corresponding approximately to the original input PCM signal.
Vector quantization coding systems have herefore not been fully developed because of the existence of several problems. For example the larger the number of bits per sample, and the larger the number of samples per group of samples or (or vector, sometimes referred to as the dimension), the larger must be the code book memory. The larger the memory is the longer is the best fit search time. For reasonably high transmission rates and signal resolution which would not degrade the resultant intelligibility of the speech signal, it had been found that the processing time of economically priced processors is excessive. In order to transmit signals at costs competitive with other encoding techniques, low resolution signals were required to be transmitted, resulting in poor speech reproduction at the receiving end of the transmission system.
One of the techniques used to increase the resolution is to utilize two code book memories in series. A coarse resolution vector code book memory lookup of a first code book memory is first conducted, followed by a second, of a second code book memory, in time sequence. The result is the provision of a high resolution code word (which term is used herein synonymously with the term table lookup entry number). However while decreasing memory size, the use of two memories slows down the lookup time, and utilizes substantial processor capacity. Thus additional processor capacity must be provided for the system, increasing its cost.
In the present invention only a single initial encoder code book memory is required, and a highly efficient search technique is used, thereby incurring minimum code book lookup time.
The resulting code book entry number is converted back to a vector signal and is compared with the primary vector signal. The difference, which constitutes an error, is scalar or vector quantized and is transmitted to the decoder in a separate channel. In the case of low traffic on the transmission line, both the code book entry and the error signal are transmitted to the decoder, resulting in a high resolution transmission rate, e.g., 32 Kb/S. However once the system has been loaded above a predetermined threshold, the error signals are not transmitted. The code words are thus transmitted at their normal rate of e.g. 16 Kb/S, i.e. at lower resolution, but doubling the capacity of the transmission line, assuming that all multiplexed signals carried by the transmission line switch to lower resolution at the same time.
Indeed for a multiplexed system the channels can be graded, with certain channels reverting to 16 Kb/S at a higher energy threshold than others. The system can also eliminate transmission of the error signals channel by channel as traffic builds up, adapting the resolution of the signals transmitted, and the capacity of the system, as traffic increases or decreases.
At the decoder the code words are translated into PCM encoded speech signals, which are modified by the quantized error signals into higher resolution signals if the quantized error signals have been received, or remain at lower resolution if the error signals have not been received.
Accordingly a high efficiency processing system has been invented which can dynamically change its transmission capacity from higher resolution signals at a lower capacity to lower resolution signals at higher capacity.
The above embodiments, as well as others to be described below, provide a highly effective vector quantized signal processing system which has been found to be cost effective, efficient, and results in received signals having high intelligibility.